1. Field of the Invention
The present invention relates to a 590 MPa grade high-strength TRIP (strain-induced transformation) cold-rolled sheet steel with excellent elongation, stretch flange formability and formability. In the present invention, the cold-rolled sheet steel encompasses not only cold-rolled sheet steels without surface treatment but also cold-rolled sheet steels which have been surface treated by electroplating, hot dipping, chemical surface treatment or surface coating or the like.
2. Description of the Related Art
The aforementioned sheet steel can be used effectively in a wide range of industrial fields such as automobiles, electricity, machines and the like, but the following explanation focuses on automobile bodies as a typical application.
The requirements for high-strength sheet steel have increased greatly due to efforts to reduce fuel costs by reducing the weight of automobile sheet steel while giving primary consideration to ensuring safety in case of collision. Recently, these requirements have been further increased in an effort to protect the environment by reducing emissions.
However, formability requirements are strong even for high-strength steel, which must have formability suited to a variety of applications. In particularly, in automobile panel and frame applications in which the steel is press formed into complex shapes, there is demand for high-strength sheet steel which has both stretch formability (ductility, i.e., elongation) and stretch flange formability [hole expandability (local ductility)].
One kind of high-strength, high-ductility sheet steel which has been developed with the aim of providing the required properties of excellent strength and ductility while reducing automobile weight and improving collision safety is TRIP (transformation-induced plasticity) steel. This TRIP steel has a mixed structure of ferrite, bainite and retained austenite with retained austenite (γR) being produced in the structure. When this steel is processed to deform at a temperature at or above the martensitic transformation start point (Ms point), it undergoes considerable elongation due to induced transformation of the retained austenite (γR) into martensite by the action of stress.
Known examples include TRIP-type composite-structure steel (TPF steel), which comprises polygonal ferrite as the matrix phase and retained austenite, TRIP-type tempered martensite steel (TAM steel), which comprises tempered martensite as the matrix phase and retained austenite, and TRIP-type bainite steel (TBF steel), which comprises bainitic ferrite as the matrix phase and retained austenite.
Of these, efforts have been made in the past to develop TPF steels which are high-strength sheet steels with good workability. For example, Japanese Patent Application Laid-open No. H02-097620 (Claims) describes that a high-strength sheet steel with good workability can be obtained by first heating to the bainitic transition temperature range and then maintaining that temperature for a specific time (“austempering”), concentrating and stabilizing the high-diffusion-constant C in the undeformed austenite so that the austenite can be retained without being transformed into martensite at room temperature.
Due to the present focus on achieving both ductility and workability as mentioned above, however, elongation and stretch flange formability need to be further improved. In particular, stretch flange formability is a property which is required for sheet steel used in automobile chassis parts and the like and for sheet steel for auto bodies which is heavily worked. Consequently, the stretch flange formability of TRIP sheet steel needs to be improved in order to promote its use in auto chassis parts and the like, for which the weight-reducing effects of TRIP sheet steel are particularly anticipated.
Therefore, a variety of research has already been done into TPF steel with the aim of providing sheet steel which has excellent formability including stretch flange formability (hole expandability) while maintaining a balance between ductility and strength from γR. For example, Japanese Patent Application Laid-open No. H09-104947 (Claims) discloses a sheet steel which, while hot-rolled, has a microstructure composed of the three phases of ferrite, bainite and γR, wherein the ratio of the occupying rate of ferrite to grain size of ferrite and the occupying rate of γR are controlled within a specific range. This is based on the finding that while increasing γR improves the strength-ductility balance and increases total elongation, this effect can be enhanced by decreasing the grain size of the γR, and in particular formability including stretch flange formability is increased when the γR is finer. The problem, however, is that the actual improving effect on stretch flange formability is small.
It has been said that a second phase consisting of γR and martensite has an effect on extension flange formability in TRIP composite structure sheet steels. From this perspective, since the amount of stress-induced transformation of γR can be controlled by means of the working temperature in particular, a method has been proposed of improving stretch flange formability by warm working TRIP steel at between 50 and 250° C. to form the γR of the second phase into fine needles.
For example, in Nagasaka, Akihiko, Koichi Sugimoto and Mitsuyuki Kobayashi, “Improving the extension flange formability of high-strength sheet steel with the transformation-induced plasticity of retained austenite,” Materials and Processes (Iron and Steel Institute of Japan, Collected Papers), CAMP-ISIJ 35 (1995), Vol. 8, pp. 556-559, the results of a study of the effects of the morphology of the second phase on warm stretch flange formability using TRIP composite structure steel (TDP steel consisting of ferrite (polygonal ferrite), bainite and γR) are reported. According to this reference, λ was higher in Type III, in which the second phase was fine and uniform, than in Type I, in which the second phase was connected (massive), but such an improvement in λ from warm working was found only when the stamping temperature Tp was raised to 150° C., and not when stamping was done at room temperature (FIG. 5).
The experimental results reported in this reference do not show an improvement effect on λ for stamping at room temperature even when the γR of the aforementioned TDP sheet steel was fine and uniform, and the improvement effect on λ was only obtained by raising the stamping temperature. Moreover, in the aforementioned reference it was also reported that the total elongation and uniform elongation of steel having γR in such a fine state were smaller than those of steel in which the second phase was connected (local elongation was greater).
Moreover, in Sugimoto, Koichi, Tsuyoshi Kondo, Mitsuyuki Kobayashi and Shunichi Hashimoto, “Warm stretch formability of TRIP composite structure steel (effects of second phase morphology-2)”, Materials and Processes (Iron and Steel Institute of Japan, Collected Papers), CAMP-ISIJ 518 (1994), Vol. 7, p. 754, reporting the results of a study of the relationship between the second phase morphology (γR) of the aforementioned TDP steel and its elongation characteristics (uniform elongation and total elongation), it is disclosed in apparent contradiction to the preceding reference that when γR is controlled as fine needles (Type III), the elongation properties at room temperature are better than those of the connected type (Type I), but when this fine needle-type γR steel is warm worked the elongation properties decline (FIG. 2).
Japanese Patent Application Laid-open No. 2004-091924 discloses that the carbon concentration in the retained austenite as the second phase (C γR) was set at or above a fixed value in a TRIP composite structure sheet steel while the proportion of lath-shaped retained austenite was increased in order to improve stretch flange formability.
Meanwhile, Japanese Patent Application Laid-open No. 2004-043908 (Claims) discloses a TPF steel comprising a matrix phase structure of ferrite and a second-phase structure of martensite and retained austenite, wherein the area rate of the second phase structure is stipulated, the minimum volume rate (Vt γR) of the retained austenite is stipulated, and the ratio of the volume rte of retained austenite in the ferrite grains (SF γR) to the aforementioned Vt γR (SF γR/Vt γR) is also stipulated.